The calcium-sensitive bioluminescent protein aequorin will be used to monitor the changes in intracellular calcium ion concentration involved in excitation-contraction coupling in vertebrate striated muscle. The kinetic properties of the aequorin-calcium reaction will be studied in vitro, and the results used to further develop a computer model of the reaction kinetics. Physiological and pharmacological studies will be continued on isolated frog skeletal muscle fibers and preparations of cardiac muscle that have been microinjected with aequorin. Efforts will be made to introduce aequorin into cells by the liposome-fusion technique. Image intensification will be used to localize sites of calcium release and uptake in aequorin-injected cells.